Method and apparatus for driving a brushless DC motor

ABSTRACT

A drive circuit for a brushless DC motor includes a switch constructed and arranged to drive the motor with a pulse signal responsive to a control signal, and control circuitry coupled to the switch and constructed and arranged to generate the control signal responsive to rotor position information from the motor so as to synchronize the pulse signal to the rotor position. A current sensing device can be used to provide the rotor position information to the control circuitry by sensing current flowing through the motor.

This application is a continuation of U.S. patent application Ser. No.10/142,255 filed May 8, 2002, now U.S. Pat. No. 6,940,235 which claimspriority from U.S. Provisional Application Ser. No. 60/290,397 filed May10, 2001, both of which are incorporated by reference.

BACKGROUND

Brushless direct current (DC) motors typically include electroniccircuitry that energizes and de-energizes electric coils (windings) inthe motor in order to make the rotor spin. Brushless DC motors arecommonly used to drive cooling fans in electronic devices such aspersonal computers (PCs). A typical brushless DC motor used in a PC ispackaged in such a way that only two terminals are accessible: apositive power supply terminal VS and a ground terminal GND (alsoreferred to as a positive and a negative rail, respectively). A thirdterminal which provides a signal that indicates the speed of the motoris sometimes accessible as well.

Cooling fans driven by brushless DC motors have traditionally been runat full speed at all times since this is the simplest implementation. Ina typical PC, this is accomplished by simply connecting the GND terminalto a power supply ground, and the VS terminal to the computer's +12 Voltor +5 Volt power supply. This is an inefficient scheme, however, sincemost electronic devices only require maximum cooling power for shortperiods of time and at random intervals. Running the fan constantly atfull speed wastes energy and generates unnecessary noise.

A recent trend is to run the fan motor at different speeds depending onthe cooling demand. One way to accomplish this is to drive the motorwith a variable voltage power supply. This is sometimes referred to aslinear fan speed control. Linear fan speed control, however, can bedifficult and expensive to implement because it requires a variablevoltage power supply. There are also problems such as those associatedwith fact that most 12 Volt fans must initially be driven with at least6 to 8 Volts to overcome the initial resistance to rotation.

Another solution involves the use of pulse width modulation (PWM). In aPWM scheme, the power supply to the motor is repetitively turned on andoff at a fixed frequency but variable duty cycle. When the power supplysignal has a relatively low duty cycle, for example 25 percent (that is,the power supply is on 25 percent of the time and off 75 percent of thetime), the motor to turns at a relatively slow speed. Increasing theduty cycle causes the motor to spin faster. Full power is achieved byleaving the power supply signal on at all times, i.e., 100 percent dutycycle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified pictorial drawing of a prior art brushless DCmotor.

FIG. 2 is a timing diagram illustrating a prior art technique fordriving a brushless DC motor at full speed.

FIG. 3 is a timing diagram illustrating a prior art technique fordriving a brushless DC motor at a reduced voltage and speed.

FIG. 4 is a timing diagram illustrating a prior art PWM technique fordriving a brushless DC motor at a reduced speed.

FIGS. 5 and 6 are timing diagrams illustrating an embodiment of a methodin accordance with the present invention for driving a brushless DCmotor.

FIG. 7 is a block diagram of an embodiment of a brushless DC motor drivecircuit in accordance with the present invention.

FIG. 8 is a block diagram of another embodiment of a brushless DC motordrive circuit in accordance with the present invention.

FIG. 9 is a flow diagram illustrating an embodiment of a method inaccordance with the present invention for starting a brushless DC motor.

FIG. 10 is a timing diagram illustrating another embodiment of a methodin accordance with the present invention for driving a brushless DCmotor.

FIG. 11 is a timing diagram illustrating yet another embodiment of amethod in accordance with the present invention for driving a brushlessDC motor.

FIG. 12 is a timing diagram illustrating a further embodiment of amethod in accordance with the present invention for driving a brushlessDC motor.

FIGS. 13 and 14 illustrate the time between tachometer pulses for athree-phase brushless DC motor and a two-phase brushless DC motor,respectively.

FIG. 15 is a flow diagram illustrating an embodiment of a method inaccordance with the present invention for driving a brushless DC motor.

FIGS. 16-19 illustrate the time every pulse, every other pulse, everythird pulse, and every fourth pulse, respectively, from a tachometer ina brushless DC motor.

DETAILED DESCRIPTION

Although prior art PWM schemes are relatively easy and inexpensive toimplement, they generates vibrations that can damage the motor and othercomponents in the system. They also generate audible noises such asticking noises. These problems are believed to be caused by stressesthat occur when the pulse width modulated power supply signal isswitched at inopportune times.

These stresses can be understood by first considering the operation of atypical brushless DC motor such as the two-phase motor shown in FIG. 1.One set of windings in the stator 50 is designated a-a′ (also referredto as phase a), and another set is designated b-b′ (also referred to asphase b). Internal electronic circuitry in the motor operates on theassumption that the power supply signal is a constant DC voltage. Thecircuitry alternately energizes phase a and phase b, depending on theposition of the rotor 52, which rides on bearing 54. When one phase isenergized, it generates a magnetic field that attracts one pole of therotor, thereby creating a torque that causes the rotor to spin. When therotor reaches a certain position, the internal circuitry switches thefirst phase off and energizes the other phase so as to generate amagnetic field that attracts the other pole of the rotor. To minimizestresses in the motor, the phases are switched when the rotor reaches aminimum torque position. The internal circuitry generally senses therotor position by using a position sensing device such as a hall-effectsensor that generates a position signal TACH as shown in FIG. 2.

When the speed of a brushless DC motor is reduced by lowering the powersupply voltage, the magnetic field created by each phase is weaker, sothe rotor spins at a lower speed. The internal circuitry has no problemswitching the phases at the proper times because it can always detectthe rotor position using the TACH signal as shown in FIG. 3. Asdiscussed above, however, it is expensive and difficult to provide avariable voltage power supply in many electronic systems.

In a prior art PWM scheme, a fixed frequency (30 Hz for example) isselected for the PWM signal. FIG. 4 illustrates a PWM power supplysignal having a 25 percent duty cycle which typically causes the motorto turn at about half speed. When the PWM signal of FIG. 4 is applied toa brushless DC motor, the internal circuitry energizes the phases asshown by the trace labeled TACH. When the TACH trace shown in FIG. 4 isat “a on” or “b on”, phase a or phase b is energized, respectively. Whenthe trace is at the midpoint “off”, neither phase is energized becausethe PWM power supply signal is off during that time. The tick marks atthe bottom of FIG. 4 indicate the minimum torque points at which thephases should optimally be switched.

Since the PWM power supply signal is free running, that is, notsynchronized to anything, the phases are energized at positions ofrandom torque, and sometime maximum torque, between the rotor and thestator. This can cause several problems. First, bearings in the motorrely on a nominal point contact between a race and a ball. The bearingsare easily damaged by high instantaneous torque which causes impactloading between the ball and race. This creates indentations known asBrinell marks in the race. Brinell marks quickly become potential sitesfor structural damage, thereby reducing the overall reliability of themotor.

Second, energizing a phase at a high torque position produces a torqueburst that causes minute flexing of the entire motor structure, therebyresulting in an audible ticking noise. The amount of noise depends onthe motor speed, the frequency of the PWM power supply signal, and theduty cycle, all of which change depending on the particularconfiguration.

Third, if the fixed frequency of the PWM power supply signal happens tobe a harmonic of the rotational speed of the motor, the windings areenergized at the same place during each revolution. This can cause themotor to shake, thereby causing further damage to both the motor andother apparatus to which the motor is attached.

One aspect of the present invention involves synchronizing pulses in apower supply signal with the position of the rotor in a brushless DCmotor. FIGS. 5 and 6 illustrate the operation of an embodiment of amethod for synchronizing pulses with rotor position in accordance withthe present invention. Referring to FIG. 5, the power supply signal VSis a pulse width modulated signal having a train of pulses with a 25percent duty cycle. Rather than generating the pulses on a free-runningbasis, however, they are synchronized to the position of the rotor. Asshown in FIG. 5, each pulse begins at a minimum torque position. Thiscauses the internal circuitry in the brushless DC motor to energize thewindings at times that minimize stress in the motor.

When the motor is energized, instantaneous torque is characterized asfollows:

$T = {{- \frac{P}{2}}L_{sr}i_{s}i_{r}{\sin\left( {\frac{P}{2}\theta_{m}} \right)}}$where:

-   -   T=Torque in Newton-meters (a negative sign means that the        electro magnetic torque acts in the direction that brings the        magnetic fields of the stator and rotor into alignment);    -   P=number of poles;    -   L_(sr)=the mutual inductance when the magnetic axes of the        stator and rotor are aligned;    -   i_(s)=current in the stator;    -   i_(r)=current in the rotor;    -   θ_(m)=actual mechanical angle between the rotor and stator.

For a given permanent magnet AC motor, P, L_(sr), i_(s) and i_(r) areconstant. This reduces the equation to T=K*sin (θ_(m)). If the phaseschange when the torque is zero, it will not cause any undo torque in thesystem. Because K is a constant, the only thing that can be controlledis θ_(m). The angle θ_(m) can be controlled when the motor is energized.In order to set the equation to a minimum, θ_(m) must be equal to zero.Since the tachometer signal is also a relative position signal, the fancan be energized with a pulse stream that is synchronous with thetachometer.

As the duty cycle of the power supply signal increases, the rotationalspeed of the motor increases. Therefore, the frequency of the pulses inthe power supply signal is increased accordingly so that the pulsesremain synchronized with the rotor position as shown in FIG. 6. The dutycycle of the VS signal in FIG. 6 is about 55 percent (corresponding toabout 75 percent speed).

FIG. 7 is a block diagram of an embodiment of a drive circuit for abrushless DC motor in accordance with the present invention. The drivecircuit 10 receives input power 12 from any suitable source, typically afixed voltage power supply. The drive circuit generates a power supplysignal 14 having a series of pulses for driving a brushless DC motor 16.The drive circuit receives rotor position information 18 from the motorto enable the drive circuit to synchronize the pulses with the rotorposition.

Different techniques can be used to determine the rotor position. If themotor has a position signal that is accessible (from a digitaltachometer for example), the drive circuit can read the rotor positionby directly monitoring the position signal.

A technique for determining the rotor position in accordance with thepresent invention is illustrated in FIG. 8. The drive circuit 16 shownin FIG. 8 includes a switch 20 (shown here as a field effect transistor)arranged to turn power to the motor on and off in response to a PWMcontrol signal PWMCTRL from control circuitry 24. A current sensingdevice 22 (shown here as a current sensing resistor) is arranged inseries with the switch to provide a current feedback signal IFB to thecontrol circuitry. Alternatively, the parasitic on resistance of theswitch could be used to sense current. By monitoring the current flowingthrough the motor, the position of the rotor can be determined. Minimumtorque positions occur when the mechanical angle between the rotor andstator (Om) is zero. At this instant, a commutation current pulse isdetected. This technique eliminates the need for a separate positionsignal from the motor.

Start-up Sequence

In prior art PWM control schemes for brushless DC motors, the powersupply signal is usually driven at full power (i.e., not pulsed) for afixed period of time at start-up, typically in the range of a fewmilliseconds to a few seconds, to allow the motor to come up to fullspeed. The power supply signal is then pulse width modulated to operatethe motor at the required speed. Since different motors have differentstart up times, the fixed period of start-up time for prior art PWMmotor drives is typically made longer than necessary to assure that itwill be long enough for the slowest starting motors. This is inefficientand generates unnecessary noise.

FIG. 9 illustrates an embodiment of a start-up sequence for a PWMcontrol scheme in accordance with the present invention. First, themotor is turned on at full power, i.e., the power supply signal isconstantly on (not pulsed), at 100. The number of motor poles isdetermined at 102. This determination can be skipped if the number ofpoles is already known. At 104, the speed of the motor is monitoreduntil it reaches a suitable speed. The motor is then driven with a PWMpower supply signal at 106.

One method for determining when the motor has reached a suitable speedin accordance with the present invention is to count the number oftachometer edges from a tachometer signal. Since a given motor typicallytakes a certain number of rotations to come up to speed, this provides arough approximation of the motor speed.

A more sophisticated technique for determining when the motor hasreached a suitable speed in accordance with the present invention is tomeasure the time between tachometer edges. Since the number of poles isknown, the motor speed can be accurately calculated based on the timebetween tachometer edges. An advantage of this method is that itoptimizes the start-up time. That is, the power supply signal isswitched from constant-on to PWM operation just as soon as the motorreaches a suitable speed.

As used herein, tachometer edge or pulse refers not only an edge orpulse in a position signal from an actual tachometer, but also moregenerally to anything that signifies events relating to the position ofthe rotor. Thus, if the current monitoring scheme described above withreference to FIG. 8 is utilized instead of a Hall-effect tachometer,instants of minimum torque would essentially be considered tachometeredges.

Steady-state Operation

FIG. 10 illustrates another embodiment of a method for driving abrushless DC motor in accordance with the present invention once themotor has started. The top trace in FIG. 10 indicates the physicalrotation of the motor where Ø1 indicates the amount of time the motortakes for a first rotation, Ø2 is for the second rotation, etc. Thesecond trace indicates the undisturbed tachometer signal which providesposition and velocity information. The third trace illustrates the PWMpower supply signal driving the motor. A and C indicate on times,whereas B and D indicate off times. The example shown in FIG. 10 is fora six-pole (three phase) motor (i.e., six “on” times per revolution).The bottom trace illustrates the actual tachometer output signal fromthe motor, taking into account the fact that the power supply signal tothe motor is being switched on and off to control the speed. The actualtachometer output signal is used to determine the amount of time ittakes the motor to complete one rotation.

The normal on time A₁ and normal off time B₁ for the first rotation arecalculated as follows:Ø1/P=A ₁ +B ₁where P is the number of poles in the motor. The duty cycle determinesthe relationship between A and B:A ₁ =DC(A ₁ +B ₁)B ₁=(1−DC)(A ₁ +B ₁)where DC is the duty cycle (percentage on time).

During the second rotation (Φ2), the PWM power supply signal is turnedon during times A1 and off during times B1. At the end of the last “on”time A1, the power supply signal is turned off for a shortened “off”time D2, and then turned on for an indeterminate amount of time until atachometer edge is detected, and then for an additional amount of timeequal to A1. As a result, “on” time C2 is longer than A1. By turning thepower supply signal on slightly earlier than needed during the lasttachometer cycle, it assures that power to the motor will be switched onbefore the tachometer edge marking the end of the complete rotation.This assures that the entire PWM power supply signal can beresynchronized at the end of each rotation. The “D” off times should beshorter than the “B” off times by as little as possible while stillallowing an adequate margin to accommodate changing rotational speeds.Using D=0.75B has been found to provide reliable results. Theresynchronization can be accomplished with suitable position sensingtechnique such as the current monitoring scheme described above.

The motor speed is controlled by varying the duty cycle DC. After acomplete revolution is completed, the duty cycle is updated, and the onand off times for the next revolution are recalculated.

The methods described herein can be used with brushless DC motors havingany number of poles, and not all poles need be utilized. That is, themotor can be driven by using fewer than all of the poles. For example,in the technique described above with respect to FIGS. 5 and 6, themotor can be driven using only phase a and leaving phase b off as shownin FIG. 11. This can be helpful in applications where high resolution isrequired at the low end of the operating range.

Another method in accordance with the present invention involves the useof multiple pulses on each phase during a single revolution. An exampleembodiment of such a technique is illustrated in FIG. 12.

Determining Number of Poles

Further aspects of the present invention involve determining the numberof poles in a brushless DC motor. The poles in a brushless DC motor arearranged almost symmetrically around the stator. However, the poles arenot spaced at exactly even intervals to assure that the motor will beginrotating at start-up regardless of the position in which the rotorstopped previously. This asymmetry causes slight variations in the timebetween tachometer edges. By measuring the time between tachometer edgesand looking for patterns, it is possible to determine the number ofpoles in the motor.

FIG. 13 illustrates the times between successive tachometer edges for asix pole (three-phase) brushless DC motor running at a steady speed. Athree-phase motor provides three tachometer pulses per revolution. Thevertical axis is the time between successive pulses in microseconds, andthe horizontal axis is a count of the tachometer edges, both positiveand negative. Data is only shown for the rising edges. For comparison,FIG. 14 illustrates the times between successive tachometer edges for afour pole (two phase, or two tachometer pulses per revolution) brushlessDC motor running at a steady speed.

A useful pattern that emerges from FIGS. 13 and 14 is that the number ofphases is equal to one plus the number of successive instances in whichthe time between tachometer pulses is less than the previous timebetween pulses. Thus, by counting the number of successive times thatthe time between pulses decreases, the number of poles in the motor canbe determined.

FIG. 15 is a flow diagram illustrating an embodiment of a method fordetermining the number of poles in a brushless DC motor in accordancewith the present invention. Beginning at 200, a counter CNTR isinitialized to zero. The time between a first tachometer pulse and asecond tachometer pulse is measured at 202 and assigned to the variableT1. The time between the second and a third tachometer pulse is measuredat 204 and assigned to the variable T2. At 206 and 208, CNTR isincremented if it is not zero. T2 is then compared to T1 at 210. If T1is not less than T2, the value of T2 is assigned to T1 at 212, and a newvalue of T2 (the next time between pulses) is determined at 204.

If T1 is less than T2 at 210, the counter CNTR is tested again at 214.Here, if the counter has a nonzero value, it is the number of phases inthe motor, so the method stops at 216. Otherwise, the counter is againset to zero at 218, the value of T2 is assigned to T1 at 212, and a newvalue of T2 (the next time between pulses) is determined at 204.

To increase reliability, the entire process illustrate in FIG. 15 ispreferably repeated a few times to confirm that the correct result hasbeen obtained. The method shown in FIG. 15 can be used even when themotor is still starting up.

In some brushless DC motors, the time between successive pulses has theopposite orientation. That is, the time between successive pulses keepsincreasing before falling back down, rather than decreasing beforerising back up. Therefore, the method illustrated in FIG. 15 ispreferably modified to also count the number of times T1 is successivelygreater than T2. Alternatively, a separate algorithm that counts thenumber of times T1 is greater than T2 can be employed when it has beendetermined that the motor being evaluated has the opposite orientation.

Another embodiment of a method for determining the number of poles in abrushless DC motor in accordance with the present invention is tomeasure the time between different numbers of pulses, thereby generatingdifferent sets of data, and then determining the data set with thelowest amount of ripple. An example embodiment of this method can beillustrated with reference to FIGS. 16-19 which shows data taken for asix pole (three phase) brushless DC motor. The data shown in FIG. 16 isthe amount of time between each successive tachometer pulse for themotor. The data shown in FIG. 17 is the amount of time between everyother successive tachometer pulse. The data shown in FIGS. 18 and 19 isthe amount of time between every third and every fourth tachometerpulse, respectively. By comparing the relative amount of ripple in thedata, it is apparent that the motor has three phases because the datataken between every third pulse has the least amount of ripple.

As discussed above, and reiterated here, a tachometer edge or pulserefers not only to an edge or pulse in a position signal from an actualtachometer, but also more generally to anything that signifies theposition of the rotor. Thus, the method described above for determiningthe number of poles in a motor can be implemented not only with anactual tachometer, but also with other methods for determining rotorposition such as the current sensing scheme described above with respectto FIG. 8. The methods described herein for determining the number ofpoles are preferably implemented with a microprocessor ormicrocontroller located in, for example, control circuitry 24 in FIG. 8.

Synthesizing a Tachometer Signal

A problem associated with pulse width modulating the power supply signalto a brushless DC motor is that the tachometer or other position sensorwithin the motor typically relies on the motor power supply foroperation. Thus, the tachometer signal can be corrupted.

Another aspect of the present invention is a method for synthesizing atachometer signal. In one embodiment of such a method, the number ofpoles is determined, the period of one rotation is determined, and thenthe rotation period is divided by the number of poles to determine asynthesized tachometer period. This can be accomplished by controlcircuitry that synchronizes the synthesized tachometer signal using anysuitable technique to initially determine the rotor position. Thesynthesized tachometer signal can then be used to synchronize pulses inthe power supply signal to the rotor position. Preferably, thesynthesized tachometer signal is periodically resynchronized using aposition sensing scheme such as a tachometer or current monitoringtechnique. The methods described herein for synchronizing and/orsynthesizing a tachometer signal are preferably implemented with amicroprocessor or microcontroller located in, for example, controlcircuitry 24 in FIG. 8.

Having described and illustrated the principles of the invention in apreferred embodiment thereof, it should be apparent that the inventioncan be modified in arrangement and detail without departing from suchprinciples. Accordingly, such changes and modifications are consideredto fall within the scope of the following claims.

1. A drive circuit for a brushless DC motor having internal circuitryoperated from a power supply signal and arranged to energize andde-energize one or more windings in the motor, the drive circuitcomprising: a switch constructed and arranged to drive the motor with apulse signal as the power supply signal responsive to a control signal;and control circuitry coupled to the switch and constructed and arrangedto generate the control signal responsive to rotor position informationfrom the motor so as to synchronize a rising edge of the pulse signalwith a minimum torque position of the motor.
 2. A drive circuitaccording to claim 1 further comprising a current sensing deviceconstructed and arranged to provide the rotor position information tothe control circuitry by sensing current flowing through the motor.